Wind Considerations in Prescribed Burning

Underburning

Weather Kit

Prescribed fires behave in a more predictable manner when wind speed and direction are steady. Onsite winds vary with stand density and crown height. Wind speed generally increases to a maximum in the early afternoon and then decreases to a minimum after sunset. The preferred range in wind speed in the stand is 1 to 3 mph (measured at eye-level) for most fuel and topographic situations.

Wind speed readings for most fire-weather forecasts are, however, taken 20 feet above ground at open locations. Wind speeds in fire-weather forecasts are the maximum expected and not the average for the day. The minimum 20-foot wind speed for burning is about 6 mph and the maximum is about 20 mph. These are the most desirable winds for prescribed burning, but specific conditions may tolerate other speeds. As a general rule higher wind speeds are steadier in direction.

Steady Winds

Relatively high winds quickly dissipate the heat of a backing fire. The result is less crown scorch than from a fire backing into a low-speed wind. In-stand wind speeds should be in the low to middle range (1 to 2 mph) when heading fires are used. With high winds, heading fires spread too rapidly and become too intense. On the other hand, enough wind must be present to keep the heat from rising directly into tree crowns.

Of greater importance than wind speed is the length of time the wind blows from one direction. Persistent wind directions occur frequently during winter, especially following passage of a cold front when winds are typically from the west or northwest. As these winds slowly shift clockwise over the next few days, they become weaker and less steady. Winds with an easterly component are generally considered undesirable for prescribed burning.

However, along the coast, sea and land breezes are often utilized. Irrespective of direction, a forecast of wind steadiness should always be obtained.

The most critical areas, with regard to fuel and topography, should be burned when wind direction is steady and persistent. Relatively easy burns can be conducted under less desirable wind conditions. Topography, and local effects such as stand openings, roads, etc. may have a bearing on favorable wind conditions and should always be considered when planning a burn.

For more information on wind, see: General Winds or Convectional Winds.

Debris Burning

Winds are stronger in open areas than they are in the forest. Because there is no overstory to protect, wind is not needed to cool the heated combustion products. However, from a smoke management standpoint, the stronger the wind the better the dispersion, provided there are no downwind smoke-sensitive areas that will be impacted. When broadcast burning, eye-level winds over 3 to 4 mph can create containment problems if a heading fire is used. With piled or windrowed debris, eye-level winds of 8-10 mph can be tolerated by adjusting the firing pattern.

Wind direction may change substantially with height, but it is these transport winds that regulate the movement of the smoke column. Moderate transport wind speeds allow a convection column to develop that exhausts the smoke high into the atmosphere where it quickly disperses with a minimum impact on ground-level air quality. Before setting a fire that will generate a convection column, however, obtain information on the existing and forecast wind profiles. If an adverse profile exists, it is likely to result in an unacceptably high spotting potential. Fire behavior characteristics are associated with various wind profiles. Once the fire has died down and smoke production is from smoldering combustion, surface wind is necessary to ensure good smoke dispersion.

For more information on wind, see: General Winds or Convectional Winds.